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Wednesday, February 5, 2014

CEMENT CONCRETE CONSTRUCTION

The cement concrete is amixture of cement, sand, pebbles or
crushed rock and water, which when placed in the skeleton of forms and allowed
to cure, becomes hard like a stone.

The cement concrete had attained the status of a major
building material in all branches of the modern construction because of the
following reasons.

I. It can be readily moulded into durable structural items
of various size and shapes at practically no considerable labour expenditure.

II.It is possible to control the properties of cement
concrete within a wide range by using appropriate ingredients and by
applying special processing techniques mechanical, chemical and physical.

III.It is possible to mechanize completely its preparation
and placing processes.

IV.It possesses adequate plasticity for the mechanical
working.

In this chapter the building construction carried out
with cement concrete will be briefly discussed.

PROPERTIES OF CEMENT CONCRETE

The cement concrete possesses the following important
properties :

I. It has high compressive strength .

II. It is free from corrosion and there is no
appreciable effect of atmospheric agents on it.

III. It hardens with age and the process of hardening
continues for a long time after the concrete had attained sufficient
strength. It is this property of cement concrete which gives it a distinct
place among the building materials.

IV. It is proved to be more economical than steel.
This is due to the fact that sand and pebbles or crushed rock, forming the bulk
of cement concrete, to the extent of about 80% to 90%, are usually available at
moderate cost. The form work which is of steel or timber, can be used
over and over again or for other purposes after it is removed.

V. It binds rapidly with steel and as it is week in tension,
the steel reinforcement is placed in cement concrete at suitable places
to take up the tensile stresses. This is termed as the Reinforced
Cement Concrete or simply R.C.C.

VI .under the following two conditions , it had a tendency
to shrink :

a. There is initial
shrinkage of cement concrete which is mainly due ot loss of water through
forms, absorption by surfaces of forms, etc.

b. The shrinkage of cement concrete occurs as it hardens.
This tendency of cement concrete can be minimized by proper curing
of concrete .

VII. It has a tendency to be porous. This is
due to the presence of voids which are formed during and after its placing.
Following two precautions are necessary to avoid this tendency :

VIII. It forms a hard surface, c arable of resisting
abrasion.

IX. It should
be remembered that apart from other materials, the concrete comes to the site
in the form of raw materials only. Its final strength it. However the
depend entirely on local conditions and persons handling it. However the
materials of which concrete is composed may be subjected to rigid
specifications.

MATERIALS USED IN R.C.C. WORK

Following materials are required for making the R.C.C work :

I. Cement

II. Steel

III. Aggregates

IV. Water.

1. Cement :
Before the introduction of ordinary Portland cement, the lime was used as
a cementing material. Most of the cement concrete work in building construction
is doen with ordinary Portland cement at present. But other special varieties
of cement such as rapid hardening cement and high alumina cement are used under
certain circumstances. The cement should comply with all the
standard requirements.

2. Aggregates :
These are the inert or chemically inactive materials which form the
bulk of cement concrete. These aggregates are bound together by means of
cement. The aggregates are classified into two categories : Fine
and coarse :

The material which s passing through BIS test sieve
no. 480 is termed as a fine aggregate. Usually, the natural rive sand is
used as a fine aggregate. But at places, where natural sand is not
available economically, the finely crushed stone may be used as a fine
aggregate.

The material which is retained on BIS test sieve
no. 480 is termed as a coarse aggregate. The broken stone is generally used as
a coarse aggregate. The nature of work decides the maximum size of
the coarse aggregate. For the slabs and walls, the maximum size of coarse
aggregate should be limited to one-third the thickness of the concrete
section.

The aggregates to be used for the cement concrete work
should be hard, durable and clean. The aggregates should be completely free
from lumps of clay, organic and vegetable matter, fine dust, etc. the
presence of all such debris prevents adhesion of aggregates and hence
reduces the strength of concrete.

3 Steel :
The steel reinforcement is generally in the form of round bars of mild
steel. The diameters of bars vary from 5 mm to 40 mm. sometimes the square bars
or twisted bars or ribbed –to steel are used as steel reinforcement. For
road slabs and such other constructions, the reinforcement may also consist
of sheets of rolled steel of suitable thickness. The hyrax which is a
steel lath may also be used as the steel. Reinforcement.

4 Water : This
is the least expensive but most important ingredient of concrete. The
water, which is used for making concrete, should be clean and free from
harmful impurities such as oil, alkali, acid etc. In general, the
water which is fit for drinking should be used for making concrete.

It may be noted that
sometimes the ingredients other than above are added in concrete to give
it certain improved qualities or for changing different physical
properties in its fresh and hardened stages. These ingredients or
substances are known as the admixtures. The addition of an admixture may
improve the concrete with respect to its strength, hardness,
workability, water-resisting power, etc. Following are the commonly used
admixtures :

Alum, aluminium
sulphate, barium oxide, bitumen, calcium chloride, coal ash, common
salt, iron oxide, lime mineral oils, organic oils, potassium
chloride, silicate of soda, tar products volcanic ashes, zinc chromate,
etc. For instance, when calcium chloride (CaCl2 ) is added as admixture, it
absorbs water from the concrete and water cement ratio falls down and can even
be brought down up to the limit of 0.25. Thus it gives quick setting concrete.
However the use of calcium chloride is not suitable for concrete with
reinforcing bars.

It is necessary to know the complete details of any
admixture before its recommendation together with the following factors :

I.Grading curves of aggregates and their respective
properties,

II.Method of construction,

III.Quantity of cement per m3 of concrete,

IV.Requirement of slump and retention.

V.Temperature variation,

VI.Type and make of cement, and

VII.Water-cement ratio.

Depending upon their respective activities in the
concrete mix, the admixtures can be classified in the following
five categories :

I.Accelerators,

II.Air entraining admixtures,

III.High range of water reducers or super plasticizers,

IV.Normal range of water reducers or plasticizers, and

V.Retarders.

It may be noted that some admixtures may have the combined
effect of the above individual activities, The popularity of
various types of admixtures in concrete is increasing rapidly because of
the following advantages available from their use :